Low entropy heat exchanger especially for use with solar gas processors

ABSTRACT

An apparatus and method for solar energy powered gas processing. There is disclosed a processor assembly for use in heating a process or feed gas by means of a convective heat exchanger which receives solar energy from a focused solar energy collector. A specialized heat exchanger is described, which maximizes the area upon which solar energy can radiate, as well as the available area for heat transfer to the flowing gas. Various modes for operating the processor assembly are described. The apparatus and method may be utilized to heat a gas to drive a heat engine. The apparatus and method also may be used to heat feed gasses to drive a gas processing reaction, such as for steam methane reformation or a reverse water-gas shift reaction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the filing of U.S. ProvisionalPatent Application Ser. No. 61/127,034 entitled Low Entropy HeatExchanger, Especially for Use with Solar Gas Processors, and BeamSpreader Useable Therewith, filed on May 10, 2008 and the entirespecification thereof is incorporated herein by reference.

Also, this application claims the benefit of the filing of U.S.Provisional Patent Application Ser. No. 61/197,922 entitled SolarEnergy-driven Reverse Water-gas Shift Reaction for Generation of Syngasfor Fuel Production, filed on Nov. 1, 2008 the contents of which alsoare incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention (Technical Field)

The present invention relates generally to systems for solar heating andprocessing of gases, particularly to heat exchangers for use in suchsystems, and more specifically to a heat exchanger for use inassociation with a solar energy collector, such as a focusing dish, toconvert solar energy into thermal energy to process gases or to drive,for example, a Brayton Engine.

2. Background Art

One of the most critical challenges confronting mankind is that ofdwindling sources of non-renewable energy. Consequently, a wide varietyof increasingly sophisticated and promising efforts have been, and are,made in the field of solar energy. Devices and methods for exploitingsolar energy as a renewable resource fall into at least two generalcategories: those attempting to convert solar energy directly intoend-use energy (such as photovoltaic electricity generation, and passivethermal heating of dwellings), and those seeking to harness solar energyas an intermediate energy source for processing feedstock into end-usefuels (e.g., methane).

There are known apparatuses and methods for exploiting solar energy toprocess feedstock gasses to generate directly useable energy and/orderivative fuels. Two examples which serve as background to the presentdisclosure are the systems and methods of U.S. Pat. No. 6,066,187 toJensen, et al., entitled “Solar Reduction of CO₂,” and U.S. Pat. No.7,140,181 to Jensen, et al., entitled “Reactor for Solar Processing ofSlightly-absorbing or Transparent Gases,” both of which name aco-inventor in common with the present application. The disclosures andteachings of these two patents are incorporated herein in by reference.

In the former '087 patent to Jensen, et al., the red shift of theabsorption spectrum of CO₂ with increasing temperature permits the useof sunlight to photolyze CO₂ to CO. The disclosed processes of the '087patent to Jensen, et al., include: preheating CO₂ to near 1800 K;exposing the preheated CO₂ to sunlight, whereby CO, O₂ and 0 areproduced; and cooling the hot product mix by rapid admixture with roomtemperature CO₂. The excess thermal energy may be used to produceelectricity, and to heat additional CO₂ for subsequent process steps.The product CO may be used to generate H2 by the shift reaction or tosynthesize methanol.

In the latter '181 patent to Jensen, et al., there is disclosed asolar-powered reactor for processing of slightly absorbing andtransparent gases to providing storable, renewable, energy through solardissociation of gas molecules. The dissociation products are theprecursors readily useable and-use liquid and gaseous fuels, such ashydrogen and methanol/ethanol. An apparatus and method using a solarconcentrator (such as a focusing trough or dish) directed at thereceiving end of a reactor are disclosed. A range of designs of reactorsfor the dissociation of gases, both those that absorb slightly in thevisible spectrum and those that are transparent in the visible and onlyabsorb in the infrared, are described.

The methods and apparatuses of the foregoing two patents, however,involve the heating of the process gases to over 2,000 degrees C.,complicating the design, and increasing construction costs, forfunctional reactor systems. It would be desirable to provide asolar-energy base system for generating useable energy, particularlyderivative storable fuels such as methane, but which does not involvesuch relatively high operating temperatures. More specifically, loweroperating temperatures. (e.g., around 800-1100° C.) might be coupledwith higher gas through-put, but at comparatively lower entropies, topermit the generation of directly exploitable energy, or for thereformation of methane as an end-use storable/portable fuel, or otherhot gas processing.

Against the foregoing background, the present apparatus and method wereconceived and reduced to practice.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and form a partof the specification, illustrate several embodiments of the presentinvention and, together with the description, serve to explain theprinciples of the invention. The drawings are only for the purpose ofillustrating a preferred embodiment of the invention and are not to beconstrued as limiting the invention. In the drawings:

FIG. 1 is a perspective view of a solar collector and gas processorassembly according to the present disclosure;

FIG. 2 is an exploded perspective view of a processor assembly accordingto the present disclosure;

FIG. 3 is a side sectional view of an assembled processor assemblyaccording to the present disclosure, showing the direction of solarenergy to be incident upon the heat exchanger component thereof;

FIG. 4 is an enlarged axial view of a foil coil heat exchanger accordingto the present disclosure, taken along section line 4-4 of FIG. 3, andhaving a substantially uniform foil baffle thickness;

FIG. 5A is an enlarged perspective view, in partial section, of aportion of the foil coil heat exchanger seen in FIG. 4;

FIG. 5B is an enlarged axial front view of a portion of the foilseparator and a portion of the corrugated foil baffle of the foil coilheat exchanger partially depicted in FIG. 5A;

FIG. 6 is an enlarged axial view of an alternative embodiment of thefoil coil heat exchanger according to the present disclosure, similar tothe embodiment seen in FIG. 4 except having a radially variable foilbaffle thickness;

FIG. 7 is a side sectional view of the processor assembly seen in FIG.3, illustrating process or feed gas flow paths according to one mode ofoperating the apparatus of the present disclosure;

FIG. 8 is a side sectional view of the processor assembly similar tothat shown in FIG. 3, illustrating process or feed gas flow pathsaccording to an alternative mode of operating the apparatus of thepresent disclosure; and

FIG. 9 is similar to the depictions of FIG. 7 or FIG. 8, showing the useof a mirror component in the processor assembly, and illustrating thetrace of a hypothetical solar ray reflecting from the mirror into thefoil coil heat exchanger.

Like numbers are used to denote like elements and components throughoutthe various drawing figure views.

DESCRIPTION OF THE PREFERRED EMBODIMENTS (BEST MODES FOR CARRYING OUTTHE INVENTION)

The present disclosure pertains to an apparatus and method for solarheating gases, such as to heat a feedstock gas to drive an engine, or toheat a gas for processing, for example for methane reformation or todrive a reverse water-gas shift. There is provided by the disclosure aprocessor by which a feed or process gas, which can be a gas relativelytransparent to much of the solar spectrum, can be effectively heated bysolar energy so that the hot gas can be harnessed to drive an engine, orfor use in the production of useable fuels.

Succinctly, the presently disclosed apparatus and method allow forcollected solar energy to be transferred by convection into a flowingfeed or process gas. Convective heating of a flowing gas is, of course,well-known in the art of heat exchangers generally, but the presentinvention enables the efficient exploitation of solar energy to heat aflowing process gas. Among advantages of the present invention is itscapability to provide a very high heat exchanger surface area tomaximize the transfer of solar power to a flowing gas. The apparatusmaximizes the area A factor in the convective heat transfer equation:

P=K A ΔT

where P is the thermal power transferred, K is the convective heattransfer constant, and ΔT is the temperature difference between the gastemperature and the temperature of the surface. The area A is maximized,yet in a manner which permits solar energy directly to heat that surfacearea by direct impingement.

The apparatus and method can be harnessed for driving a heat engine, forexample a Brayton Engine (e.g., in the form of a Capstone 30 engine),with the energy received from a solar energy focusing dish. There isdisclosed hereby a gas processor apparatus including a heat exchangercapable of transferring 70 kW of solar energy into heat, into arelatively high flow gas, to drive for example a Brayton Engine. Thelow-entropy processor assembly, including a beneficial foil-coil heatexchanger, is depicted generally in the drawing figures and will bedescribed. It is contemplated that one embodiment of the thesolar-powered apparatus and method according to the present disclosureis capable, for example, of heating 200 cubic feet per minute of gas(Cp=7.5 cal/mole deg) approximately 500 degrees C. in a single pass. Thesingle-pass methodology nevertheless can result in final temperatures ofabout 900-100° C. Such a gas temperature range is not only excellent forBrayton engines, but also for, for example, methane reforming.

An apparatus generally according to the present disclosure has beenoperated to drive a Capstone 30 generator. A suitable gas flow rateaccording to one possible system according to this disclosure is 150 to170 SCFM, rather near a target 200 CFM at 70 kW. The incident solarpower on a typical run might be near 50 kW.

The present apparatus and method also are well-suited for chemical gasprocessing, such as reforming of methane. That utility was as asecondary use in prior U.S. Pat. No. 7,140,181, but it has beendetermined that the design of that disclosure is “overkill” forreforming methane. The presently disclosed simpler, higher through-put,lower entropy production design is more advantageous for heating at toaround 900° C. Also, the corrugated foil coil hot body exchangerdisclosed hereby lends itself to catalytic reforming.

Attention is invited to FIG. 1. The apparatus and method include the useof a solar collector 10 configured to “gather” solar energy. Many knowntypes of solar energy collectors, or others devised hereafter, may beadapted for use according to the present apparatus and method. The solarcollector 10 depicted in FIG. 1 is a mirrored dish type collector, inwhich a plurality of mirrors are mounted according to convention forredirecting gathered solar energy toward a focal point. Mirroreddish-type collectors are well-suited to the practice of the invention;however, other types of solar energy collectors, such as a field ofheliostats of known or hereafter-developed configurations, may also beadapted for use in the present apparatus and method.

Referring still to FIG. 1, the present apparatus also has a gasprocessor assembly 20 which is used with the solar collector 10. The gasprocessor assembly 20, which is described further herein, is disposedwith the solar collector 10 in a manner such that solar energy collectedby the solar collector 10 is reflectively re-directed toward the gasprocessor assembly 20. Thus, the processor is mounted to be at or nearthe “focal point” of the solar collector 10 in use.

The major components of a possible and basic gas processor assembly 20according to the present disclosure are shown in the exploded view ofFIG. 2. The assembly 20 includes a housing 24, which may be a hollowcylinder of stainless steel of (for example) about eight to eighteeninches in diameter. A heat exchanger 50 according to the presentdisclosure is configured for coaxial disposition within the interior ofthe housing 24. In this embodiment, the heat exchanger 50 also defines acylinder with an exterior radius just slightly less than the insideradius of the housing interior 27. The heat exchanger 50 thus may beinserted to the appropriate axial location well within the housing 24,and securely but removably attached by brackets 25 or other suitablemeans. In a completed installation, the exterior of housing 24 (exceptits proximate end which is covered by a quartz pane 28) iswell-insulated (insulation not shown in the drawing figure) to reducethe loss of heat energy from within the processor assembly 20. Thehousing 24 may be stainless steel, rather than any very-high temperaturealloy, because the gas flowing through the processor assembly 20 is coolcompared to the high temperatures obtained in the material of the foilcoil exchanger 50 itself.

The proximate end 29 of the housing 24 is the aperture or mouth end,which is that end which faces toward the solar collector 10. The solarcollector 10 directs collected energy toward the proximate end 29 of theprocessor assembly 20, for passage through the proximate end of thehousing 24. When assembled, the processor assembly 20 includes at leastone quartz pane 28 for closing the proximate end of the housing 24.Quartz pane 28 is configured according to principles known in the art,and permits transmission there-through of solar energy directed from thesolar collector 10. More than one quartz pane may be provided inparallel relation, so as to provide an outer pane on the exterior of theprocessor assembly 20 and a second pane interiorly to the first. It isknown that a quartz pane causes some losses (e.g., about 10%) intransmitted solar energy, but quartz is suited for the high temperaturesinvolved while permitting operation of the processor by solar power. Thequartz pane 28 configured according to known principles is mounted on orabout the proximate end of the housing 24 according generally toconvention, using suitable gaskets or O-rings and attachment means (notshown in FIG. 2). The mounting of the pane 28 is a sealed attachment toprevent leakage of gas past the pane 28 and in or out of the interior ofthe processor 20.

The distal end of the processor assembly 20 is sealably closed using aback panel 34, which may be a stainless steel disc of dimensionsappropriate to seal the distal end. Back panel 34 is securely mounted tothe housing 24 using known gasket and attachment means (not shown) toprevent leakage of gas into our out from the interior of the processor20. The entire processor assembly is constructed to operate with aninterior pressure, which pressure will depend upon the gas processreaction to be exploited. Thus, the design of the processor assembly 20will include from known information a construction to permit operationat elevated interior pressures; the Reverse Water-Gas Shift Reaction,for example, may be promoted by pressure conditions well exceedingatmospheric pressure.

In one embodiment, there is optionally provided within the processor 20near the distal end thereof a mirror 40. Mirror 40 is fashioned from ahigh-temperature material and is polished to reflect incident solarenergy. The mirror 40, if deployed, is situated axially inwardly fromthe back panel 34, so as to be contained within the distal end of thehousing 24.

As seen in FIG. 2, the processor 20 is provided with at least one firstor forward port 30 through the wall of the housing 24, via which gas maybe moved into or out from the interior of the housing 24. At least onesecond rear port 32 is provided closer to the distal end of theprocessor 20.

Reference is invited to FIG. 3, showing fundamental aspects of theapparatus and method. FIG. 3 depicts in cross-section an assembledprocessor assembly 20. When assembled, the processor assembly has aforward chamber 42 and a rear chamber 44 within the interior of thehousing 24, the chambers 42, 44 being separated by the intermediatelyinstalled heat exchanger 50. The proximate end 29 of the processorassembly 20 is closed against gas leakage by the quartz pane 28, whilethe distal end is closed by the back panel 34. The processor assembly ispositioned in relation to the solar collector 10 so that solar rays aredirected toward the proximate end of the processor assembly 20. Solarenergy, indicated by the small solar ray directional arrows of FIG. 3,is directed to enter the processor assembly 20 as they pass through thequartz pane 28. The solar rays are transmitted through the forwardchamber 42 to be incident upon the first face 52 of the heat exchanger50. The heat exchanger 50 is heated to high temperature by the incidentsolar energy, with the heat being conducted axially through theexchanger 50 from the vicinity of the first face 52 toward the exchangerrear face 54. There is thus created a temperature gradient from thefirst face 52 toward the rear face 54 of the exchanger. As shall bedescribed further herein, feed or process gas is effectively heated bypassing axially from either of the chambers 42 or 44 through thesuperheated exchanger 50 to the other of the chambers 44 or 42. Theapparatus for heating a gas thus includes the heat exchanger 50, and thecollector 10 for reflectively directing solar energy toward the heatexchanger thereby to heat the exchanger, wherein the feed gas flowsaxially through the heat exchanger to be heated thereby.

Incident solar energy is transmitted primarily by conduction from thefront face 52 throughout the body of the exchanger 50. Solar rays alsomay reflectively “bounce” along the axial flow channels defined throughthe exchanger (as described further herein), enhancing the directradiant transfer of solar energy to the exchanger 50. Additionally, thefeed or process gas flows axially through the exchanger 50 (in eitherdirection, as described more fully herein), such that solar heat whichhas been transferred into the gas also is restored to the foils 56, 58of the exchanger by means of convection. The latter is particularlybeneficial in the embodiment of the apparatus shown in FIG. 8.

The proportion of heat transfer by conduction versus by radiationdepends upon the dimensions of the openings in the face of theexchanger, the gas flow rate, and the thickness of the foils 56, 58 inthe exchanger 50. In-foil conductive heat transfer along the exchanger50 depends upon, among other factors, the composition of the exchangerfoils 56 and 58, and their thicknesses. The solar radiation heatingalong the axis of the exchanger is a function of, among other things,the size of the openings in the front face 52 of the exchanger, and theoptics (e.g., F factor or degree of focused convergence) of the incomingsolar rays. Applying known physical formulae, it is possible tocalculate the in-foil conduction and axial radiation transfer values,permitting some customization of the exchanger construction to optimizeits operation.

Such a configuration for the processor assembly offers benefit of fully“ingesting” the incident solar energy. This advantage is complemented bythe benefit of a modest-diameter aperture at the mouth end 29 of theprocessor, thus reducing energy losses from back-irradiation.

Combined reference is made to FIGS. 4, 5A, and 5B. In one embodiment ofthe apparatus and method, the heat exchanger 50 is a “foil coil.” Thefoil coil heat exchanger 50 includes a rolled foil separator 56 and arolled corrugated foil baffle 58, wherein the corrugated foil baffle 58is wound with the separator 56 such that the heat exchanger is definedby the foil baffle 58 being situated between consecutive windings of thefoil separator 56.

The corrugated foil baffle 58 and the separator 56 are fabricated fromthin, flexible strips of suitable metal alloy, such as copper allow ornickel alloy, capable of withstanding the high temperatures resultingfrom the solar energy impinging the exchanger 50. By way of example, thefoil strips may have a thickness from about 0.002 inch (2.0 mil) toabout 0.007 inch (7.0 mil). The foil strips may have an axial dimensionof from about 8.0 cm to about 14.0 cm, and an overall length (prior toheat exchanger fabrication) of from about 90 m to about 110 m. The foils56, 58 preferably are rolled together to fabricate a heat exchanger 50in the shape of a cylinder having a diameter of from approximately 16 cmto approximately 30 centimeters. This mode of fabrication allows for avariety of heat exchanger diameters, however, and all dimensions areoffered only by way of example. The axial width of the foil coil heatexchanger 10 corresponds to the axial width of the foil strips fromwhich it is fabricated, e.g., preferably approximately 10 cm. Theprocessor assembly 20 and exchanger 50 are fully scalable according togas processing design demands.

The corrugated foil baffle 58 is fabricated so to present in the axialdirection a series of alternating “peaks” and “valleys,” as seen inFIGS. 4, 5A and 5B. The corrugated baffle 58 thus preferably defines, inaxial profile, a continuous series of triangles (with adjacent trianglesinverted with respect to each other) of height p and base dimension d(FIG. 5B). This zig-zag profile creates between the peaks a series ofvalleys through which the feed or process gas flows during the practiceof the invention. The corrugated baffle 58 may be manufactured byrunning a foil strip between a pair of complementary counter-rotatinggears (not shown) mounted so that the teeth of one gear mesh with thegrooves of the other. The teeth of the gears have dimensionscorresponding to the desired dimensions of the triangular corrugationsto be produced in the foil baffle 58. By feeding the foil stock betweenthe intermeshing teeth of the rotating gears, the gear teeth impress andfold the foil material to corrugate the foil strip into theconfiguration shown in greatest detail in FIGS. 5A and 5B. In oneexemplary embodiment of the apparatus, each triangle in the corrugatedfoil baffle 58 has an altitude p in the range of from approximately 40mil to approximately 140 mil, seen in FIG. 5B. The base d of eachtriangular corrugation typically is in the range of from about 100 milto about 150 mil. It is understood, however, that all dimensions areoffered by way of example, not by strict limitation. An advantage of thepresent invention is that the characteristics of the heat exchanger 50may be customized by varying the size and shape of the corrugations inthe foil baffle 58. It is further to be understood that the corrugationsare not necessarily triangular; alternative suitable embodiments may befabricated with other axial profiles, including rounded wave-likecorrugations defined by a series of alternating convex and concavesemicircles, or other axial profile shapes defining parallel axialvalleys through which feed gas may flow axially through the exchanger50.

Referring to FIGS. 4 and 5, it is seen that the foil coil heat exchanger50 preferably is fabricated by placing a strip of corrugated foil baffle58 into contact with a correspondingly sized (as to length and axialwidth) uncorrugated foil separator 56. As suggested by FIGS. 5A and 5B,the apices of the corrugation triangles contact an adjacent foilseparator 56. The combination of the foil baffle 58 and adjacent foilseparator 56 defines a series of parallel channels (in this example,triangular channels) across the axial width of the foils 56, 58. Oncethe foil separator 56 and the corrugated foil baffle 58 have been placedin parallel registration, they are rolled or wound together to definethe heat exchanger seen in axial view in FIG. 4, in which the foilseparator 56 and the foil baffle 58 are spiral wound to definealternating windings of separator and baffle.

Referring to FIG. 4, the heat exchanger 50 in the preferred embodimentthus features alternating coaxial (but spiral) windings of foilseparator 56 and corrugated baffle 58. The foils 56, 58 are woundtogether with the corrugated baffle 58 “inside” the foil separator, sothat the exterior surface of the completed heat exchanger 50 is definedby the smoothly curved, non-corrugated, separator 56. The spirally woundheat exchanger 50 thus preferably defines a cylinder shape, suitable forplacement within the interior of the processor housing 24 with theoutside layer of foil separator adjacent to the interior wall of thehousing 24, as suggested by FIGS. 2 and 3. The heat exchanger seen inFIG. 4 thus has about twenty courses or “windings” of foil separator 56,each winding of the separator serving to separate serially consecutivecourses or windings of the corrugated baffle 58. The embodiment of theheat exchanger 50 depicted in FIG. 4 accordingly has about nineteenwindings of the corrugated baffle 58, the exterior winding being that ofthe separator 56; the baffle 58 and separator then alternate as oneproceeds toward the central axis of the exchanger 50.

The foil separator 56 in certain embodiments (such as might be used forgas heating to drive a Brayton Engine) where the processing temperatureof the heat exchanger 50 does not exceed about 900° C. may be a thincopper foil, or nickel-coated copper foil. Copper has a comparativelylower melting temperate (approximately 1070° C.), but is a goodconductor of heat energy to promote even heating of the exchanger 50.Likewise, under similar operating specifications, the corrugated foilbaffle 58 may be pressed from a thin copper or nickel-covered copperfoil. Nickel is generally more inert, but may also serve as a mildcatalyst for certain hot gas processing reactions, so a nickel-coatedfoil 56 and/or 58 may be beneficial in some applications. However, wherethe heat exchanger 50 is to be solar heated to temperatures above 900°C., the foils 56 or 58 should be comprised of a superalloy. Thus, in apreferred embodiment of the apparatus adapted for use in gas processing,for example to reform methane or to drive a reverse water-gas shiftreaction, the foil separator 56 and the corrugated foil baffle 58 areboth made of superalloy, for example, Haynes® 214® nickel superalloyavailable from Haynes International, Inc., of Kokomo, Ind., USA. Such asuperalloy has a melting temperature of about 1355-1400° C., permittingthe exchanger 50 to be solar heated to adequate processing temperatureswithout physical failure.

Still referring to FIGS. 4 and 5B, it is seen that in the heat exchanger50, each course or winding of the corrugated foil baffle 58 has a radialthickness dimension corresponding generally to the altitude p of theconstituent corrugate triangles. In the embodiment of the exchanger 50shown in FIG. 4, the radial thickness dimensions of the successivewindings of baffle 58 are substantially equal (proceeding radially fromor toward the center of the heat exchanger). The radial pattern andcross-section of the alternating separator 56 and the foil baffle 58thus is generally uniform across the radius of the heat exchanger 50.The flow of processing gas axially through the channels defined by andbetween the separator and baffle windings accordingly is not a functionof the flow's radial distance from the central axis of the exchanger.

As indicated by FIG. 3, during the practice of the invention the solarenergy may be directed so as to focus in the central portions of thefirst or front face 52 of the exchanger 50. Accordingly, it may bedesirable to provide for enhanced cooling of that radially centralportion of the exchanger, which generally is the portion of theexchanger which obtains the highest temperatures from the incident solarradiation. Such cooling is provided by the flow of feed or process gasthrough the exchanger. It may be desirable, therefore, to promotecooling gas flow by providing larger gas flow channels through thecentral region of the heat exchanger 50. A benefit of the presentlydisclosed apparatus and method is that the axial gas flowcharacteristics of the heat exchanger 50 can be tailored or customizedto be a function of the flow distance from the central axis of theexchanger. Larger gas flow channels may be supplied by providingrelatively larger distances between “peaks” of the corrugated baffle 58(i.e., comparatively longer triangle bases dimensions d) and/orrelatively greater radial thicknesses of the baffle 58 (i.e.,comparatively longer triangle altitude dimensions d). Variableadjustment of these values can be accomplished by regulating the“tightness” of the rolling or winding of the foils 56, 58 as a functionof the distance from the exchanger's central axis.

FIG. 6 illustrates how the axial flow characteristics of the heatexchanger 50 can be beneficially and selectively adjusted, in aparticular heat exchanger, to be a function of the radial distance fromthe central axis. The heat exchanger 50 embodiment of FIG. 6 exhibitsdenser foil windings nearer the outside of the exchanger, whereexchanger temperatures are apt to be the lowest and the need for coolinggas flow comparatively less. But because the solar energy may be focusedin the central region of the front face of the exchanger 50 (FIG. 3),resulting in increased need for cooling gas flow, the windings of theseparator 56 and the corrugated foil baffle 58 are proportionally“looser,” or less dense, than at the exchanger's radially outer regions.In sum, in the embodiment of FIG. 6, each winding of the corrugated foilbaffle 58 has a radial thickness dimension, but the radial thicknessdimensions of the windings of the baffle 58 vary as a function of theradial distance from the exchanger's central axis, for example, as oneproceeds radially outward from the center of said heat exchanger 50.Specifically, in one possible embodiment, the radial thicknessdimensions of successive windings of the baffle 58 progressivelydecrease, proceeding radially outward from the center of the exchanger50.

Such varying of the characteristics of the baffle 58 as a function ofradial distance from central axis may be accomplished by adjusting thecompressive force applied to the foils 56, 58 during their spiralwinding. At the inception of winding—that is, where the foils 56, 58 arefirst rolled to define the center of the exchanger 50—a relativelyrelaxed winding force and pressure is applied to the pair of adjacentfoils 56, 58. Consequently, the baffle 58 maintains its original axialthickness (as corresponding generally to nearly the initial manufacturedaltitude p). The first three or four (for example) courses or windingsof the foils 56, 58 are maintained with the originally applied windingcompression and force, such that the distance between adjacent windingsof the separator 56 remains unchanged at about the maximum (initial)distance. Then, and as suggested by evaluation of FIG. 6, the appliedcompressive winding force is progressively increased at a selected rate,causing the corrugated foil baffle 58 partially to collapse. As the foilbaffle 58 is “smashed” under the increased compression, the trianglebase dimension d of the corrugate triangles (FIG. 5B) increases, and thealtitude dimension p decreases correspondingly. Increasing compressivewinding force causes progressively greater collapse of the baffle 58, inturn resulting in progressively decreasing separation distances betweenconsecutive foil separators 56. As seen in FIG. 6, therefore, the baffle58 is comparatively flattened in the outer portions of the exchanger,causing the exchanger coil to be comparatively denser in those portions.

Advantageously, using known physics formulae, the density of theexchanger coil can be selectively varied by selectively varying thewinding force applied to the foils 56, 58 during rolled fabrication ofthe exchanger 50. Greater compression applied to the foils 56, 58 duringrolled winding results in decreased baffle height p, as the baffles are“smashed” to reduce the absolute volume of the flow channels, whichvolume is a function of the baffle's axial profile. Normally, theexchanger 50 is roll fabricated to have its least dense foil windings atand near its central axis (to promote cooling gas flow axially past thefoils 56,58), and with density increasing progressively as somepredetermined function of the distance from the central axis. However,one skilled in the art will appreciate that the applied winding forcecan be varied in many modes during fabrication of the exchanger, so thatcoil density may be varied to either to decrease or increase atdifferent distances from the axis. One skilled in the art also willdesign, by application of known principles of fluid dynamics, anexchanger 50 according to the present disclosure to adjust density tomaximize the available heat transfer area presented by the exchanger,but without unduly compromising gas flow through-put.

Additionally, the density of the foil coil within the exchanger 50affects the distance which incident solar rays are able to penetrate theinterior of the exchanger 50. The less dense the foils 56, 58 arewound—that is, the greater the distance between windings of the foilseparator 56 and the more “open” the corrugated baffle 58 -the furtherthe solar rays are able axially to penetrate the exchanger 50. Increasedaxial penetration may promote a relatively more consistent radiantheating of the exchanger foil coils 56, 58 along the axial dimension,decreasing the steepness of the thermal gradient along the exchanger 50.Beneficial results are increased efficiency in feed gas heating andprolonged exchanger life. Notably, however, the foils 56, 58 are not tobe wound too “loose,” as a critical factor in the function of theexchanger is the total area presented by the foils 56, 58 for contactwith the passing gas. More and denser windings increase the availablefoil area for heat transfer to the gas, thus better driving the heattransfer equation, in which foil area is to be optimized, but in view ofthe need also not to unduly hinder gas flow.

Returning attention to FIG. 3, we again note that the processor assembly20 includes the exchanger housing 24 within which the heat exchanger 50is disposed. The housing 24 and the first face 52 of the exchangerdefine in part the first forward chamber 42. The solar collector 10directs solar energy to the first face 52 of the exchanger 50. Thehousing 24 and the second face 54 of the exchanger 50 define in part thesecond rear chamber 44. During the practice of the method or operationof the apparatus, feed or process gas flows from one of the chambers 42or 44 to the other 44 or 42 of the chambers, as will now be furtherdescribed.

FIG. 7 shows that there is disposed at least one (in many preferredembodiments, a plurality) a first forward port 30 through the housing24, and through which the feed gas enters the forward chamber 42 enroute to the heat exchanger 50. A pump 35 or some other pressure orvacuum means is in operative or fluid communication with the forwardport 30 to move gas through the heat exchanger 50 from the forwardchamber 42 to the second rear chamber 44. Because the foil coils in theheat exchanger 50 have been heated to high temperature by the solarenergy incident thereon, the feed or process gas is heated as it passesthrough the exchanger 50. The heated gas emerges into the rear chamber44 where it optionally may be subjected to further processing (such as,for example, temperature quenching to prevent back-reactions). Theresulting heated gas is tapped from the rear chamber 44 via one or morerear ports 32; the hot processed gas is then delivered downstream forfurther processing or storage as desired.

In the embodiment of FIG. 7, the operation is a “hot side feed,” in thatthe feed gas is introduced to the hot side (i.e., front face 52) of theexchanger 50, and flows through the exchanger 50 with the temperaturegradient—that is, the gas flow is down-gradient (from hot toward cold;the exchanger front face 52 being hotter than the rear face 54). Gasflow is indicated in FIG. 7 by the large curling directional arrows. Inthis “hot side feed” regime, there is the benefit of cooling the hotfront face 52 of the exchanger with the first pass of the unheated feedgas. Cooling the hot side of the exchanger reduces the thermal stress onthe foils 56, 58 of the exchanger, but may do so by somewhatcompromising entropy and process efficiency. Care in operating ahot-side flow regime must always be exercised, lest the front face 52 ofthe exchanger 50 be melted by excessive temperatures.

An advantage of the present apparatus is that the foil coil exchanger50, while not only transferring convectively the solar energy into theflowing gas, can be operated in either a forward-flow or a reverse-flowflow mode. Thus alternatively, and as seen in FIG. 8, the method andapparatus may be run in a desirable “cool side feed” mode. In thisembodiment, there is a second or rear port 32 through the housing 24,through which the feed gas enters the second or rear chamber 44 en routeto the heat exchanger 50. Gas flow is indicated in FIG. 8 by the largecurling directional arrows. A pump 35 or other pressure or vacuum meansmoves the gas through the heat exchanger 50 from the second chamber tothe first or forward chamber 42. In this embodiment, the gas flowsagainst the thermal gradient, that is, from the cooler portions of theexchanger 50 toward the hotter portions neared the front face 52. Such alower-entropy flow regime more efficiently increases the temperature ofthe flowing gas, but with the disadvantage of maximizing the thermalstress in the foil coils 56, 58 of the exchanger 50 (because the hottestportion of the exchanger is cooled by gas that has been heated bytransit through the exchanger). After transiting the exchanger 50, thehot gas exits the forward chamber 42 via one or more forward ports 30,after which the hot gas is further processed or transported according tothe particular application.

In the embodiment of FIG. 8, suited for “cool side feed” operation,there very preferably is provided a second, interior quartz pane 36. Thesecondary or interior pane 36 is recommended to deflect away from theprimary, sealed, pane 28 as much as possible of the hot gasses exitingthe front face 52 of the exchanger 50. The quartz shielding pane 36transmits the entering solar energy. The shielding pane 36 need not havea sealed engagement with the housing 24, but its radius is as large aspracticable to prevent hot gas from impinging the cool, sealed, primaryquartz pane 28.

Reference is made to FIG. 9, which illustrates the advantageous butoptional use of a mirror 40 near the distal end of the processorassembly 20. Use of the mirror 40, which is disposed and configured inany suitable manner to reflect incident solar energy back toward theexchanger 50, provides for a “second pass” of solar energy back to theexchanger 50. This second pass of reflectively delivered energy promotesgreater and more even heating of the exchanger along its axis, as thecooler rear face 54 is the recipient of the reflected energy. The use ofa distal mirror 40 is particularly indicated in combination with aprocessor assembly 20 which allows a significant amount of solar energyto transit axially the exchanger 50. For example, and as discussedhereinabove with respect to the radially variable density foil coilexchanger 50 of FIG. 6, the foils 56, 58 of the exchanger may be“loosely” wound near the exchanger's central axis, with the result thata substantial quantity of solar light incident on the front face 52 maypass through the interstices of the exchanger and emerge past the rearface 54 thereof.

The small directional arrows of FIG. 9 show a hypothetical path of asolar ray through this embodiment of the apparatus. The ray passesthrough the quartz pane 28 and enters the exchanger 50, e.g., betweenadjacent windings of the foil separator 56. The ray may reflect one ormore times from the surfaces of the foils 56 or 58, exit the exchanger50, and impinge upon the mirror 40. It then is reflected back toward theexchanger 50, and its impingement upon any surface of the foil coils 56,58 contributes to the further heating of the exchanger. The apparatusaccordingly may include such a mirror 40 disposed in the rear chamber 44in confronting relation with the back face 54 of the heat exchanger 50,whereby solar energy passing through the exchanger and into the rearchamber 44 is reflectively redirected toward the rear face 54 of theexchanger to heat the exchanger. In some embodiments of the apparatus,up to 20-30% of the solar energy incident upon the front face 52 of thecoil passes completely through the exchanger 50; use of the mirror 40thus may allow the recovery of up to about 70-80% of the 20-30% of theenergy that otherwise might be lost to the process.

FIG. 5A illustrates an optional aspect of the apparatus and method. Thedisclosed configuration of the foil coil exchanger 50 permits the facileplacement of a catalyst 59 in the interstices of the foils 56, 58, ifindicated for certain processing reactions. The desired reaction iscatalyzed as the gas flows through the exchanger and past the containedcatalyst 59. For example, in the use of the apparatus for thereformation of methane, a reformer catalyst 59 can be loaded or lodgedin predetermined quantities in some or all the interstices or flowchannels defined in the foils 56, 58. A suitable powder-type catalystfor methane reformation might be, for example, the Nickel-based SteamReforming Catalyst No. 45465 available from Alfa Aesar Company of WardHill, Mass., USA. In the optional instance where a powder catalyst isloaded in the exchanger 50, swatches of appropriately sized and gaugednickel gauze can be disposed across the front and rear faces 52, 54 toprevent spillage of catalyst from the interior of the exchanger 50.Alternatively, a layer of catalyst may be fixedly deposited onto some ofthe surfaces of the foils 56, 58.

The apparatus and method works well for solar applications because itsdesign transfers concentrated heat (bright solar spot) into a feed orprocess gas by providing a large gas contact surface area. The use ofcopper foils 56, 58 in the body of the exchanger 50 is effective,because its high conductivity spreads the heat from the very hot frontsurface 52 toward the back face 54 (that is not being directlyirradiated). Consequently, the gas passing through the foil coil heatexchanger has copious surface for heat transfer. This is expressed bythe convective heat transfer equation:

P=K A ΔT

where P is the thermal power transferred, K is the convective heattransfer constant, and ΔT is the temperature difference between the gastemperature and the temperature of the surface. The convective heattransfer constant K is a function of the type of gas and its velocity,and may be calculated using known concepts and formulae. The convectiveheat transfer rates typically are between 10 and 35 watts/m² per degreeK, depending on the gas, the surface, and the temperature.

From the heat transfer equation it can be determined that to transfer50,000 watts at a ΔT of 100° K requires near 20 m² of exchanger surfacearea. If a foil 56, 58 is 10 cm wide, near 100 meters of foil arerequired for this case (not 200 meters, because each foil 56, 58 has twosides).

We have operated a case where 44 kW of heat was transferred into flowingair. The foil coil exchanger was 10 cm wide, with the wound foils 56, 58being 95 meters long. While it is difficult to assign a ΔT for suchexemplary run, it is known that the temperature varied between 1060degrees C. at it hottest point (center of the front face 52 to onlyabout 400° C. at the rear face 54, near the outer edge). The exit gaswas heated to an average temperature near 700° C. for the important partof the run. The homogeneity of the gas flow is unknown for the case, butits total flow rate was 147 SCFM.

It is of interest to look at the range of likely average ΔT and heattransfer parameters for the experimental run. If it is assumed that thegas flow was homogeneous, the product of KΔT would be near 2600watts/m². The following Table 1 shows compatible sets of K's and ΔT'sfor the example run. These are all in the probable range and verify theeffectiveness of the disclosed foil coil exchanger design.

TABLE 1 K (w/m2 K) ΔT (K) 15 173 20 130 25 104

In order to explore the advantages of flowing the gas in against thethermal gradient in the exchanger (FIG. 8), we built a simplified modelof the performance of the foil coil heat exchanger 50 by dividing thecoil into five slices with fixed temperatures that we believeapproximated those obtained experimentally. The flow channels weretriangular with a base of 0.100 inches (100 mil) and an altitude of 0.04inches (40 mil) (FIG. 5B). The results of the model are shown in Table 2below. The model shows that much higher temperatures are achieved usingthe “cool side feed” mode, whereby gas flows from the cooler portion ofthe exchanger 50 to the hot end, than when it flows from the hot frontface 52 toward the cooler rear face 54.

TABLE 2 Average metal Temp in slice Entering gas (centigrade) temp C. ΔTCentigrade Gas Flow Hot to Cold Slice 1 1000 25 195 Slice 2 750 220 175Slice 3 575 395 81 Slice 4 450 476 −13 Slice 5 380 463 −40 ExitTemperature 423 Gas Flow Cold to Hot Slice 1 380 25 71 Slice 2 450 96 87Slice 3 575 183 120 Slice 4 750 303 173 Slice 5 1000 475 263 ExitTemperature 739

The apparatus and method may be used, for example, to reform methane.The elevated temperatures obtained in the foil coil exchanger 50 arerealized from solar energy, and are sufficiently high to drive thenatural gas reformation step of the known steam methane reformingprocess. At high temperatures (700-1100 ° C.) and in the presence of ametal-based catalyst (e.g., nickel), steam reacts with methane to yieldcarbon monoxide and hydrogen. The appropriate feedstock gases are passedthrough the exchanger 50 according to the forgoing disclosure, and thereformation reactions thereby driven forward.

Some of the chemical reactions that can take place in the course ofmethane reformation are:

C_(n)H_(m)+n H₂O→n CO+(m/2+n) H₂

and

CO+H₂O→CO₂+H₂

The produced carbon monoxide can combine with more steam to producefurther hydrogen via the water gas shift reaction. Of course, otherreactions (some undesirable, like coke formation) can take place iflocal conditions are favorable. The first reaction is endothermic while,the second reaction is exothermic. Additional fundamentals regardingmethane reformation are found in, for example, U.S. Pat. No. 7,087,651to Lee-Tuffnell, et al., and U.S. Pat. No. 6,312,658 to Hufton, et al.

The disclosed apparatus and method may also be adapted to exploit theReverse Water-Gas Shift Reaction (rWGSR). The rWGSR reaction is given byequation:

CO₂+H₂→CO+ΔH₂O H=+9 kcal/mole (38.9 kJ/mol)

The rWGS reaction accordingly may be exploited to generate CO from CO₂.The CO may then be used as feedstock for further processing into useablefuels. For example, produced CO can be feed directly into a knownFischer-Tropsch synthesis system to generate synthetic fuels. TheFischer-Tropsch synthesis is a relatively complex network of bothparallel and series chemical reactions; it is a carbon-chain buildingprocess whereby CH₂ groups are attached to the carbon chain, andinvolves the catalytic reaction of H₂ and CO to form hydrocarbon chainsof various lengths. A principle by-product of the Fischer-Tropschprocess system is water.

This reaction is endothermic, and occurs at relatively reducedtemperatures in the presence of certain catalysts (e.g., element 59 inFIG. 5A), including CuO, ZnO and or Al₂O₃, at temperatures of 300° C. orgreater. Nevertheless, even at 400° C. the equilibrium constant (Kp)driving the reaction is only about 0.1, and even at somewhat highertemperatures Kp remains of an order perhaps approaching unity. Also, theequilibrium constant is somewhat pressure-dependent, but the value ofK_(p) is likely approximately 0.3 in the pressure range of about 0.5 MPato 2 MPa. The present apparatus and method are capable, however, ofsufficient heat transfer to eliminate the need for catalysis during therWGSR; the use of expensive and sometimes unpredictable catalysts shouldbe considered optional in the presently disclosed method.

The catalyzed rWGSR reaction always is most limited, however, by theforward water-gas shift reaction, as the reverse (or in this instancethe “reverse-reverse”) reaction normally also is catalyzed. One possiblemode for minimizing the back-reaction is to condense (and centrifugallyseparate) H₂O from the flowing mix subsequent to partial reaction,followed by a recycling loop. (Separation of CO₂ from the produced gasstream will always be necessary.) Further, the undesirable back-reactioncan be suppressed by strategic placement of the catalysts within thereactor (forward location in the exchanger 50), coupled with rapidquenching by immediately downstream heat exchangers, such as forexample, as disclosed in U.S. Pat. No. 7,140,181 to Jensen, et al.,entitled “Reactor for Solar Processing of Slightly-absorbing orTransparent Gases.” While cooling the gas stream immediately after therWGS Reaction shifts the equilibrium toward the promotion of CO₂ and H₂,at the reduced temperature the forward water-gas shift reaction isrelatively slow, and in the absence of an immediately available andsuitable catalyst may not occur at all.

Because in the endothermic rWGS Reaction the amount of equilibriumconversion of carbon dioxide into carbon monoxide is directlyproportional to temperature, very high reactor temperature in the foilcoil exchanger 50 is desirable. This makes the rWGSR an attractivecandidate for the use of concentrated radiant energy to drive thethermochemical process.

The disclosed method and apparatus thus may be adapted to drive the rWGSReaction. Concentrated sunlight, as from a suitable mirrored collectiondish 10, is directed through the quartz window 28, into the processorassembly 20, and upon the foil coil heat exchanger 50. The spiraled andknurled foils 56, 58 in the heat exchanger 50 provides the heat transferand also lends itself to providing desirable catalysis, such as byincluding Cu/alumina or Cu/silica, or CuO in combination with a Nicoating on foils 56, 58.

Hydrogen is mixed with carbon dioxide to produce an H₂: CO₂ feed gasmixture in a ratio of approximately 1:1. Alternatively, it may bedesirable to “overdose” the H₂, to boost the ratio to near 3:1 to reducethe amount of unconverted CO₂ in a first pass. Other mixture ratios maybe used as well to optimize the performance of the rWGS reactor.

As seen in FIG. 8, the mixture is introduced into the processor assembly20 via the rear input port 32, and passes through the solar-heated Culoaded g-alumina catalyst (or other suitable catalysts) heat exchanger50 in the housing 24. The temperature within the reaction zone withinthe heat exchanger 50 is about 450° C. to about 700°, so that CO₂ isconverted to CO in the exchanger 50 and chamber 42 according to the rWGSReaction. There preferably is provided a radially variable flow channelcross section through the crinkled foil, for example by using deepercorrugations near the center of the coil comprising the exchanger 50, asseen in FIG. 6. This helps to modulate the strong heating near thecenter of the apparatus. Smaller flow channels nearer the circumferenceof the exchanger 50 coil help to “catch up” the outside heating.

Quenching (rapid cooling) of the process gas is important for CO₂splitting. The quencher is merely a heat exchanger placed immediatelydownstream of the reaction (where radicals involved in the undesirableback-reactions, including especially the forward gas-water shiftreaction, are favored) to cool the gas below about 450° C. andpreferably to about 200° or so.

Accordingly, the produced CO and H₂O may be vented through the forwardport 30 and immediately quenched in a cooling exchanger (not shown) inthe physical absence of any rWGSR catalyst materials. Heat surrenderedfrom the product gasses during quenching can be used to providepre-heating to the feedstock CO₂ entering the input port, if desired.

The apparatus may be operated in a recycling mode, with close to 100%equilibrium conversion. Alternatively, “boosting” conditions can beprovided, as previously explained, to drive the reaction to completeconsumption of one of the reactants (e.g., CO₂) by overloading the other(H₂), followed by product extraction (preferably the produced water).The carbon dioxide may be separated by isothermal compression andfurther cooling or by other known methods. Produced CO₂ preferably isreturned to react in the processor 20 via the rear input port 32. Theremaining carbon monoxide and hydrogen may be fed directly to a syngasprocessor. In an alternative process, the hydrogen may be separated (atleast partially) from the gas mixture by membrane filtration, andreturned as feedstock to the rWGS Reactor processor assembly 20.

Although the invention has been described in detail with particularreference to these preferred embodiments, other embodiments can achievethe same results. Variations and modifications of the present inventionwill be obvious to those skilled in the art and it is intended to coverin the appended claims all such modifications and equivalents. Theentire disclosures of all patents cited above are hereby incorporated byreference.

1. An apparatus for heating a feed gas comprising: a heat exchangercomprising: a rolled foil separator; and a rolled corrugated foilbaffle, wherein said corrugated foil baffle is rolled with saidseparator such that said heat exchanger comprises said foil bafflesituate between windings of said foil separator; and a collector forreflectively directing solar energy toward said heat exchanger therebyto heat said exchanger; wherein a feed gas flows axially through saidheat exchanger to be heated thereby.
 2. An apparatus according to claim1 wherein said foil separator and said foil baffle are spiral wound todefine alternating windings of separator and baffle.
 3. An apparatusaccording to claim 1 wherein said foil separator or said corrugated foilbaffle comprises nickel superalloy.
 4. An apparatus according to claim 1wherein said foil separator or said corrugated baffle comprises copperalloy.
 5. An apparatus according to claim 2 wherein each winding of saidcorrugated foil baffle has a radial thickness dimension, and whereinsaid radial thickness dimensions of said windings are substantiallyuniform.
 7. An apparatus according to claim 2 wherein each winding ofsaid corrugated foil baffle has a radial thickness dimension, andwherein said radial thickness dimensions of said windings vary as afunction of the radial distance from the center of said heat exchanger.8. An apparatus according to claim 7 wherein said radial thicknessdimensions progressively decrease proceeding radially outward from thecenter of said heat exchanger.
 9. An apparatus according to claim 1further comprising an exchanger housing within which said heat exchangeris disposed; said housing and a first face of said exchanger defining afirst forward chamber, wherein said collector directs solar energy tosaid first face of said exchanger; said housing and a second face ofsaid exchanger defining a second rear chamber; wherein said feed gasflows from one of said chambers to the other of said chambers.
 10. Anapparatus according to claim 9 further comprising: an inlet through saidhousing through which feed gas enters said first chamber en route tosaid heat exchanger; and a pump to move feed gas through said heatexchanger from said first chamber to said second chamber.
 11. Anapparatus according to claim 9 further comprising an inlet through saidhousing through which feed gas enters said second chamber en route tosaid heat exchanger; and a pump to move feed gas through said heatexchanger from said second chamber to said first chamber.
 12. Anapparatus according to claim 9 further comprising a mirror disposed insaid second chamber in confronting relation to said second face of saidheat exchanger, whereby solar energy passing through said heat exchangerinto said second chamber is reflectively redirected toward said secondface of said heat exchanger to heat said exchanger.